Heating a recording medium placed within a magnetic field to a temperature above its Curie point by irradiating it with a semiconductor laser beam causes changes in polarization rotation characteristic. This effect is utilized to record signals. As a matter of course, the lower the temperature of the recording medium due to low ambient temperature, the more thermal energy is required for raising the recording medium to the Curie point.
Japanese Unexamined Patent Publication No. HEI 3-256234 discloses an optical recording/reproducing system which includes a light source comprising a semiconductor laser, a control circuit for controlling the output power of the semiconductor laser, temperature sensing means disposed in the proximity of a recording medium, and a detecting circuit which detects the temperature from the temperature sensing means. The output from the detecting circuit is used to control the output from the semiconductor laser during erasure and recording. In other words, the temperature of the recording medium is measured for controlling the intensity of the recording laser beam in accordance with the measured temperature of the recording medium.
Japanese Unexamined Patent Publication No. SHO 54-83404 shows a recording medium which is pre-heated by irradiation with a laser beam, for example.
The intensity of the laser beam at a surface of a magneto-optic disc or record medium required to raise the disc temperature to the Curie point changes at a rate of, for example, -0.03 mW/.degree.C. This means that when ambient temperature is low and, therefore, the disc temperature is low, particularly, during start-up, a large writing energy is necessary.
In determining the power of a semiconductor laser for use in the magneto-optic recording/reproducing system, the energy required at the disc surface and the energy coupling efficiency in the optical system must be considered. For example, assuming that the required energy at the disc surface is 10 mW and the coupling efficiency is 33.3%, the laser output should be 30 mW. Assuming further that the temperature of the disc at the start of recording changes over a range of, for example, 30.degree. C., the required energy at the surface may change over a range of about 1 mW, and, accordingly, the semiconductor laser output must be able to vary over a range of about 3 m.
Accordingly, in the system of the aforementioned Japanese Unexamined Patent Publication No. HEI 3-256234, the recording laser beam intensity in low temperature conditions must be increased by 10% or more. Since the semiconductor laser must provide an output higher than its rated output, its reliability is lowered.
In the system shown in the aforementioned Japanese Unexamined Patent Publication No. SHO 54-83404, there is a danger that the pre-heating of the recording medium might raise the temperature of the medium above its Curie point so that recorded information could be erased.